Color: Improve legibility of ANSI foreground colors

On colored theme backgrounds, take great liberties to modify the ANSI foreground color so it remains legible.

3 files changed, 21 insertions, 18 deletions

diff --git a/res/about/help.gmi b/res/about/help.gmi
index 3c332f81..a25f556e 100644
--- a/res/about/help.gmi
+++ b/res/about/help.gmi
@@ -423,6 +423,7 @@ Sometimes these codes are used for things like colored ASCII art. However, Lagra
 The "ANSI escapes" setting controls which ANSI escape codes are enabled:
 
 * "FG Color" enables changes to text foreground color.
+* "BG Color" enables changes to text background color.
 * "Font Style" enables changing the font style: bold, italic, or monospace. (These correspond to codes 1, 2, and 11.)
 
 A warning banner is displayed if any codes are detected on a page. This helps you be aware of potential visual artifacts, like text color that is being forced to black regardless of the page background color. Click on the warning to dismiss it on a per-site basis.
diff --git a/src/ui/color.c b/src/ui/color.c
index adbe444b..0b927fd6 100644
--- a/src/ui/color.c
+++ b/src/ui/color.c
@@ -460,6 +460,10 @@ iColor rgb_HSLColor(iHSLColor hsl) {
     return toColor_(init_F4(r, g, b, hsl.a));
 }
 
+float luma_Color(iColor color) {
+    return 0.299f * color.r / 255.0f + 0.587f * color.g / 255.0f + 0.114f * color.b / 255.0f;
+}
+
 const char *escape_Color(int color) {
     static const char *esc[] = {
         black_ColorEscape,
@@ -851,26 +855,23 @@ void ansiColors_Color(iRangecc escapeSequence, int fgDefault, int bgDefault,
                 break;
         }
     }
-    /* Ensure legibility if only one of the colors is set. */
-    /* TODO: Force darkening of the background color, unless it is also specified. */
-#if 0
-    if (bg.a == 0 && !equal_Color(fg, get_Color(fgDefault))) {
-        if (delta_Color(fg, get_Color(tmBackground_ColorId)) < 64) {
-            const iHSLColor fgHsl = hsl_Color(fg);
-            iHSLColor legibleBg = get_HSLColor(tmBackground_ColorId);
-            if ()
-            bg = rgb_HSLColor(bgHsl);
-        }            
+    /* Ensure legibility if only the foreground color is set. */
+    if (fg.a) {
+        const iHSLColor themeBg = get_HSLColor(tmBackground_ColorId);
+        const float bgLuminance = luma_Color(get_Color(tmBackground_ColorId));
+        if (bgLuminance > 0.4f) {
+            float dim = (bgLuminance - 0.4f);
+            fg.r *= 0.5f * dim;
+            fg.g *= 0.5f * dim;
+            fg.b *= 0.5f * dim;
+        }
+        if (themeBg.sat > 0.15f && themeBg.lum >= 0.5f) {
+            iHSLColor fgHsl = hsl_Color(fg);
+            fgHsl.hue = themeBg.hue;
+            fgHsl.lum = themeBg.lum * 0.5f;
+            fg = rgb_HSLColor(fgHsl);
         }
     }
-#endif
-    /*
-    if (!bg_out || get_HSLColor(tmBackground_ColorId).lum > 0.5f) {
-        fg.r /= 2;
-        fg.g /= 2;
-        fg.b /= 2;
-    }
-    */
     if (fg.a && fg_out) {
         *fg_out = fg;
     }
diff --git a/src/ui/color.h b/src/ui/color.h
index 6602b226..0b3f8bed 100644
--- a/src/ui/color.h
+++ b/src/ui/color.h
@@ -233,6 +233,7 @@ struct Impl_HSLColor {
 
 iHSLColor       hsl_Color       (iColor);
 iColor          rgb_HSLColor    (iHSLColor);
+float           luma_Color      (iColor);
 
 iHSLColor       setSat_HSLColor     (iHSLColor, float sat);
 iHSLColor       setLum_HSLColor     (iHSLColor, float lum);